Much effort is currently being devoted to the development of optical networking systems as an alternative to electronic-based networks. In this emerging technology, pulses of light are used instead of currents of electrons to carry out such diverse networking functions as data transmission, data routing, and other forms of data communication and processing. Such functions are achieved with a number of discrete components, but integral to virtually all developing optical networking systems are optical-waveguide structures that are used to guide light being propagated from one location to another. For example, in one specific application that is being aggressively developed, optical waveguides are used to confine and carry optical signals in conformity with a dense wavelength division multiplexed (“DWDM”) protocol. Such a protocol increases the amount of information carried with individual optical signals by multiplexing discrete wavelength components, thereby increasing the effective bandwidth that may be accommodated with the optical networking system.
To illustrate the use of optical waveguides in such systems, FIG. 1 provides a cross-sectional view of a typical optical-fiber waveguide 100. The waveguide includes two principal components—a core 104 through which the light is propagated and a cladding layer 102 that acts to confine the light. To ensure that the light is confined, the core 104 is usually surrounded completely by the cladding layer 102, which also generally has a lower refractive index (“RI”). The difference in refractive indices of the core 104 and cladding layer 102 permit light to be confined by total internal reflection within the core 104. FIG. 1 illustrates the concept of total internal reflection with an exemplary light ray 106, with the confinement angle θc representing an upper limit on angles at which the light can be incident on the core/cladding interface without leakage.
As more wavelength components are incorporated into optical-waveguide channels within DWDM systems, there is a corresponding increase in demand for optical components to perform routing, switching, add/drop, and other functions. A variety of photonic components have the capacity to perform such functions, including, for example, filters, modulators, amplifiers, couplers, multiplexers, cross connects, arrayed waveguide gratings, power splitters, star couplers, and others. As optical networking technology matures, however, one goal is to integrate various photonic components monolithically onto a single structure, such as a silicon-chip or glass substrate.
A number of efforts have been made at such development, but attempts to integrate optical waveguides and photonic components onto a single chip have faced significant challenges. Some approaches have attempted to modify techniques for monolithic integration of electronic components, but have encountered a variety of difficulties. These difficulties often arise from fundamental differences between photonic and electronic applications. For example, the scale of photonics applications is much greater than the scale for electronics applications, sometimes as much as an order of magnitude. This difference in scale results in a need to deposit much thicker layers in photonics applications. This increased thickness has resulted both in cracking of structures because of increased stresses and in much greater variations in uniformity of the structures. In addition, techniques for monolithic integration of electronic components have been sharply focused on optimizing the dielectric constant of materials because of its importance in electronic applications. In contrast, photonic applications are instead sensitive to optical characteristics of materials, such as its refractive index. It has often been found that the methods and materials used for producing structures in electronic applications simply do not meet the optical requirements of photonic applications.
One prior-art technique that has been widely used in producing optical waveguides is flame hydrolysis. This technique is not only very costly, but has, in practice with large substrates, been found to produce structures with poor uniformity. Other techniques have been used in attempts at mitigating thermal strain by separately depositing a lower cladding layer, over which optical cores are formed, and subsequently depositing an upper cladding layer over and between the optical cores. One specific technique that has been used in such efforts is plasma-enhanced chemical-vapor deposition (“PECVD”). An example of an optical-waveguide structure formed using PECVD is shown in FIG. 2.
The cross sectional view of the optical waveguide structure 200 provided in FIG. 2 shows four optical cores 206. Light is intended to travel through each optical core 206 in a direction orthogonal to the page. The optical cores 206 are formed over an undercladding layer 204, which is itself formed over a substrate 202. The uppercladding layer 208 has been deposited with PECVD. Use of PECVD techniques is known to produce layers having significant levels of hydrogen impurities, which leads to undesirable nonuniformities in refractive index in the cladding layers. The negative effect of such nonuniformities is further exacerbated by the nonconformal nature of PECVD deposition since portions of the cladding layer may be very thin near parts of each core. Moreover, as shown in FIG. 2, such nonconformal deposition can lead to the formation of voids in the cladding layer that interfere with optical transmission and permit unsatisfactory propagation losses.
There is accordingly a persistent need for improved methods and systems for manufacturing optical waveguides with cladding layers that simultaneously meet stringent refractive-index requirements, are resistant to cracking, and are amenable to efficient use in low-cost production environments.